Light strings having lights connected electrically in series are well known, especially around the holidays when such light strings are used for decorative purposes. Generally, the lights in the string are electrically in series, rather than in parallel. One particular drawback to these types of light strings is that when a light bulb is removed from the socket, the entire series is rendered inoperable. Each light bulb within its respective socket completes the electrical circuit, so when a light bulb is removed or becomes loose, a gap is created in the circuit and electricity is unable to continue to flow through the circuit. When a light bulb is inserted back into the socket or the loose bulb reseated, it completes the circuit, thus allowing electricity to flow uninterrupted.
A number of known light units and light strings attempt to address this issue of rendering a light set inoperable due to a missing or loose bulb. For example, U.S. Pat. No. 6,257,740 issued to Gibboney, Jr., discloses a basic light unit that allows current to flow in the absence of a bulb. More specifically, Gibboney, Jr. discloses a switch mechanism that comprises a pair of relatively long, centrally-located spring terminals in a light unit. Each spring terminal is connected to a wire terminal at an interior wall of the light unit and extends inwards to the center of the light unit. When a bulb is absent from the light unit, the two spring terminals are in contact with one another, allowing current to flow through the light unit and to other light units in a light set. When a specially-adapted bulb is inserted into the light unit, the bulb separates the two spring terminals, breaking the electrical contact point, routing current to the bulb filament. As such, Gibboney, Jr. teaches that when a bulb is removed, the contacts spring inward towards the center of the light unit, to where the bulb was previously located.
In another example, U.S. Pat. No. 6,609,814 issued to Ahroni, discloses a light unit with a centrally-located mechanical switch and shunt element adapted for use with a non-conventional flat-wire light set. However, most decorative light strings utilize a twisted-pair wiring convention for which the Ahroni design cannot easily be adapted.
One of the drawbacks to the light units discussed above and of other similar mechanical designs, is a lack of reliability. For example, over time, memory effects present in the spring terminals may cause switch failure. For designs such as those described above, the spring terminals may be relatively long, with long moment arms and with the spring terminals often being integral to the wire terminals. Because bulbs are removed infrequently from any individual light unit, the spring terminals tend to be in a compressed state for long periods of time. When a bulb is ultimately removed, or becomes loose, the spring terminals move towards their original position of contact, but the spring terminals may not move all the way back to the original contact position due to the extended period of time spent in the compressed, or tensioned, position.
In addition to memory effects, further unreliability comes from movement of the switch elements within the socket. When bulbs are inserted or removed, switch contacts and supports may be dislodged or otherwise moved from their original operational positions, causing the switch to fail.
Another drawback of such designs is the relatively large area of electricity conducting material exposed when a bulb is removed. When a bulb is removed, electricity flows through the centrally-located spring terminals which span the inside diameter of the light unit. If a foreign object is inserted into the light unit when the bulb is removed, but the light set powered, the risk of electric shock is great.
In addition to known mechanical solutions to the problem of missing or loose bulbs, electrical solutions also exist. For example, some light sets use a pair of back-to-back zener diodes located in a light unit and electrically in parallel with the bulb. When a bulb is dislodged from its socket, the voltage potential across the diodes is larger than the threshold voltage of the diodes, causing the diodes to conduct.
The primary drawback to light units utilizing such electrical solutions is the high cost of the electrical components. Other drawbacks include heating of the electrical elements in the socket, complexity of design, custom manufacturing requirements and rigid tolerances.
Therefore, what is needed in the industry is an improved light unit that not only allows current to flow through the light unit when a bulb is loose or removed, but also addresses the reliability, safety and cost issues as described above.